Piezomagnetic monitoring in the South Pacific region

Data from an array of piezomagnetic stations in the South Pacific Islands indicate that noise limitations on piezomagnetic stress monitoring reported for California (Mueller andJohnston, 1981;Johnstonet al., 1984) are probably pessimistic since they seem to include significant diurnal residual. Raw station differences could be significant at the 1.0 nT level out to distances of several hundred kilometres. Two large earthquakes have occurred within the network, and no coseismic anomalies were observed. However, changes in observed magnetic field for several stations within 200 km of the earthquakes indicate that stress propagation effects are observable prior to the events, and that these effects dominate the simple elastic effects previously investigated which accompany the stress drops for moderate to large earthquakes.